One of the CIA’s most important secrets in the war on terrorism was hiding in plain sight, on a leafy residential street along a busy set of train tracks in Romania’s capital. There, tucked in the basement of a government building, the CIA ran a clandestine prison, former U.S. intelligence officials said.

For years, the building — codenamed Bright Light — housed some of the CIA’s most important terror suspects, including Khalid Sheik Mohammad, the mastermind of 9/11. Even after the detainees were shipped off to Guantanamo Bay in 2006 and reports about the prison began to surface, the Romanian government repeatedly denied any knowledge of its existence.

A joint investigation by The Associated Press and German public television, ARD Panorama, however, located the former prison and unearthed details of the facility where harsh interrogation tactics were carried out.

The Romanian prison was part of a network of so-called black sites that the CIA operated and controlled overseas in Thailand, Lithuania and Poland. All the prisons were closed by May 2006, and the CIA’s detention and interrogation program ended in 2009.